1. Field of the Invention
The present invention relates to an optical miniaturized module, optical pickup device and optical disk device, for use in recording information on and reproducing information from an optical storage medium such as an optical disk.
2. Description of the Background Art
Conventionally, the three-beam method and the push-pull method are primarily known as the tracking servo methods for optical disk devices. When such methods are employed for an optical disk device, there occurs an offset upon recording, reproduction or access. The offset may also occur when the optical disk is tilted. As a way of compensating such an offset, a differential push-pull (DPP) method has been proposed. In recent years, the DPP method has been employed for an optical disk device such as a digital versatile disk (DVD) player that records information in high density.
In the DPP method, push-pull signals are detected from reflected lights of a main beam that is the primary light (hereinafter, referred to as “main beam M”) and of two sub beams formed to follow main beam M (hereinafter, referred to as “sub beam A” and “sub beam B”). The detected values are subject to operation to generate a tracking error signal in the DPP method. Sub beams A and B are arranged before and after main beam M (for example, sub beam A is a preceding beam and sub beam B is a succeeding beam with respect to main beam M) in a direction along the track on the surface of the optical storage medium. Further, the push-pull signal obtained from main beam M has a phase inverted from those of the push-pull signals obtained from sub beams A and B. With the DPP method, the push-pull signal obtained from main beam M and the push-pull signals obtained from sub beams A and B are subject to operation to effect control to remove the offset of the tracking error signal.
An optical storage medium of a phase change method, used for a DVD player or the like, has a recorded portion and an unrecorded portion that differ in reflectance of light from each other. Thus, for example when preceding sub beam A moves from the recorded portion to the unrecorded portion, there occurs a state where the unrecorded portion is irradiated with the preceding sub beam A while the recorded portion is irradiated with the succeeding sub beam B. In such a state, only the detected signal of sub beam A experiences a change by an amount corresponding to the change of the reflectance. Using the detected signals of both the preceding and succeeding beams for the operation can reduce the deficiency in removal of the offset of the tracking error signal by the changed amount of the reflectance. Accordingly, when the DPP method is applied to the phase-change type optical storage medium, it is necessary to carry out an operation by detecting the signals of both sub beams A and B arranged before and after main beam M on the surface of the optical storage medium.
The applicant has proposed in Japanese Patent Laying-Open No. 2001-273666 an optical pickup device provided with an optical miniaturized module that can employ the DPP method. FIGS. 5A and 5B show the optical miniaturized module disclosed therein. FIG. 5A is a schematic cross sectional view of the optical miniaturized module. The optical miniaturized module includes a semiconductor laser 105 as a source of light, a three-beam diffraction grating 106 for dividing the light, a composite prism 107 for separating the laser light emitted from the light source and its reflected light from each other, a hologram element 109 for dividing the reflected light, and a photodetector 110.
Lased light 119 from semiconductor laser 105 is divided by three-beam diffraction grating 106 into main beam M and two sub beams A and B. These three beams pass through a polarization beam splitter (PBS) surface 107a formed at composite prism 107, and are directed via a ¼ wave plate 108 to a collimate lens (not shown). In FIG. 5A, sub beams A and B are not shown for the sake of simplicity of the drawing. Reflected light 120 from an optical storage medium in the form of an optical disk passes through ¼ wave plate 108, is reflected at PBS surface 107a and a reflective mirror surface 107b, and is incident on hologram element 109, where it is diffracted to enter photodetector 110.
The light emitted from semiconductor laser 105 passes through PBS surface 107a in the state of linearly polarized light in the X direction (P polarization) shown in FIG. 5A. It is then transformed into a circularly polarized light at ¼ wave plate 108, to irradiate the optical disk. The reflected light from the optical disk reenters ¼ wave plate 108, where it becomes a polarized light in the Y direction (S polarization) and is reflected at PBS surface 107a. As such, lased light 119 and reflected light 120 are separated from each other at PBS surface 107a. 
FIG. 5B illustrates the optical miniaturized module as seen through from above. Three-beam diffraction grating 106 is arranged approximately at the center of the optical miniaturized module, and hologram element 109 is arranged to its side. Composite prism 107 is formed to cover three-beam diffraction grating 106 and hologram element 109.
FIG. 6 is a plan view showing the positional relation of the hologram element and the laser light incident on the hologram element based on a conventional technique. Hologram element 109 includes a first parting line 51 that is defined in the X direction optically corresponding to the radial direction as the optical disk rotates, and a second parting line 52 that is defined in the Y direction optically corresponding to the track direction. First and second parting lines 51 and 52 partition hologram element 109 into three areas of d area 64, e area 65 and f area 66. With hologram element 109 being in a circular shape in a plane, first parting line 51 is defined to correspond to the diameter of the circle at the perimeter of hologram element 109, and second parting line 52 is defined to correspond to the radius of the circle of hologram element 109 and to be perpendicular to first parting line 51.
Main beam M is incident on a main beam M incident area 45. Main beam M incident area 45 is arranged in an approximately circular shape in a plane, coaxial with the circle at the perimeter of hologram element 109. Sub beam A is incident on a sub beam A incident area 46, which is arranged offset from main beam M incident area 45 in the Y direction. Sub beam B is incident on a sub beam B incident area 47, which is also arranged offset from main beam M incident area 45 in the Y direction. Sub beam A incident area 46 and sub beam B incident area 47 each intersect first and second parting lines 51 and 52. That is, each of these sub beam incident areas is arranged to include respective parts of the three partitioned areas of hologram element 109.
FIG. 7 schematically illustrates photodetector 110 for detecting reflected lights and an operation unit 190 for performing operations on signals obtained from the photodetector. Photodetector 110 receives reflected lights in three rows. They include a 0th-order diffracted light incident on the row indicated by an arrow 204, a +1st-order diffracted light incident on the row indicated by an arrow 205, and a −1st-order diffracted light incident on the row indicated by an arrow 206. Photodetector 110 includes light receiving portions 121-126 for receiving the light diffracted in the +1st-order direction by the hologram element, and light receiving portions 127-129 for receiving the light diffracted in the −1st-order direction. Here, signals output from light receiving portions 121-129 are represented as S121-S129, respectively. Although the 0th-order diffracted light is not shown in FIG. 5A, it occurs in a small amount. Thus, in FIG. 7, portions on which the 0th-order diffracted light impinges are shown in the row indicated by arrow 204.
In FIG. 7, a reference character has been allotted to each portion on which laser light impinges in the following manner. The first letter of the reference character indicates whether it is a transmitted light or a diffracted light. Specifically, “5” represents the +1st-order diffracted light, “6” represents the −1st-order diffracted light, and “4” represents the 0th-order diffracted light (transmitted light). The second letter indicates whether it is main beam M or sub beam A or B. “M” represents main beam M, and “A” and “B” represent sub beams A and B, respectively. The third letter indicates where in the polarization hologram the light was divided. “d” represents the light diffracted in the d area. “e” represents the light diffracted in the e area, and “f” represents the light diffracted in the f area. For example, of the impinging portions of the laser light, the one denoted by “5Md” indicates the +1st-order diffracted light of main beam M, which was diffracted in the d area of the polarization hologram. The 0th-order diffracted light is denoted by only the first two letters, such as “4M”.
In FIG. 7, of the reflected light of main beam M, the +1 st-order diffracted light diffracted in d area 64 of the hologram element (see FIG. 6) is detected at a gap portion between the neighboring light receiving portions 123 and 124. The −1st-order diffracted light is detected at light receiving portion 128. Further, of the reflected light of main beam M, the −1st-order diffracted light from the e area of the hologram element is received at light receiving portion 129, and the −1st-order diffracted light from the f area is detected at light receiving portion 127.
As to sub beam A, the +1st-order diffracted light diffracted in the e area is detected at light receiving portion 122, and the +1st-order diffracted light diffracted in the f area is detected at light receiving portion 126. As to sub beam B, the +1st-order diffracted light diffracted in the e area is detected at light receiving portion 121, and the +1st-order diffracted light diffracted in the f area is detected at light receiving portion 125. The signals detected by photodetector 110 for the lights divided by the hologram element are processed in the following manner.
A focus error signal (FES) is calculated by the following expression (1).FES=S123−S124   (1)
A tracking error signal (TES4) may be obtained from the following expression (2), based on the push-pull method using S129 and S127. However, from the above-described reasons, an operation method based on the DPP method is primarily employed, where push-pull signals TES4, TES(A) and TES(B) are obtained from main beam M and sub beams A and B, respectively, and then, TES5 is calculated as shown by the following expression (3).
                              TES          ⁢                                          ⁢          4                =                              S            ⁢                                                  ⁢            129                    -                      S            ⁢                                                  ⁢            127                                              (        2        )                                                                                    TES                ⁡                                  (                  A                  )                                            =                              (                                                      S                    ⁢                                                                                  ⁢                    122                                    -                                      S                    ⁢                                                                                  ⁢                    126                                                  )                                                                                                        TES                ⁡                                  (                  B                  )                                            =                              (                                                      S                    ⁢                                                                                  ⁢                    121                                    -                                      S                    ⁢                                                                                  ⁢                    125                                                  )                                                                                                        TES                ⁢                                                                  ⁢                5                            =                                                TES                  ⁢                                                                          ⁢                  4                                -                                  k                  ⁡                                      (                                                                  TES                        ⁡                                                  (                          A                          )                                                                    +                                              TES                        ⁡                                                  (                          B                          )                                                                                      )                                                                                                                          =                                                (                                                            S                      ⁢                                                                                          ⁢                      129                                        -                                          S                      ⁢                                                                                          ⁢                      127                                                        )                                -                                  k                  ⁢                                      {                                                                  (                                                                              S                            ⁢                                                                                                                  ⁢                            122                                                    -                                                      S                            ⁢                                                                                                                  ⁢                            126                                                                          )                                            +                                              (                                                                              S                            ⁢                                                                                                                  ⁢                            121                                                    -                                                      S                            ⁢                                                                                                                  ⁢                            125                                                                          )                                                              }                                                                                                          (        3        )            
In the above expression (3), a constant k is for compensation of a difference in intensity of the main and sub beams. For example, in the case where the relative ratio in light intensity of the reflected lights of main beam M and sub beams A and B for calculation of TES5 is a:b:c, then k is obtained by k=a/(2b). Further, at the time of reproduction of an optical disk recorded with bit information, a phase contrast of S129 and S127 may be detected to thereby obtain TES6 by a differential phase detection (DPD) method, as in the following expression, which is indicated for reference.TES6=S129−S127
A recorded information signal (RF) is obtained by the following expression (4).RF=S127+S128+S129   (4)
Operation unit 190 in FIG. 7 schematically shows circuits for performing the operations indicated by the expression (3) above. A subtracter 194 calculates TES4, and subtracters 191 and 192 calculates TES(A) and TES(B), respectively. An adder 193 calculates TES(A)+TES(B), which is multiplied by constant k by an amplifier 195. A subtracter 196 calculates TES5 by the DPP method.
As described above, in order to apply the DPP method to an optical pickup device for the phase-change type optical storage medium, it is necessary to detect two sub beams formed before and after main beam M in the track direction of the optical storage medium. Further, in an optical miniaturized module based on the conventional technique using the DPP method, as shown in FIG. 7, four light receiving portions in total are required for detecting the reflected lights of two sub beams A and B, i.e., light receiving portions 122, 126 for detection of sub beam A and light receiving portions 121, 125 for detection of sub beam B. This increases an area occupied by the photodetector, hindering downsizing of the optical miniaturized module. The increase in number of photodetectors increases the cost as well.
Further, since calculation of signals TES(A) and TES (B) involves four light receiving portions 121, 122, 125 and 126, the number of calculators increases, resulting in complicated circuit configuration.